Wireless communication systems are widely used to provide various types of communication services such as voice and data. The wireless communication systems are multiple access systems capable of sharing available radio resources and supporting communication with multiple users. For example, the radio resources are time, frequency, codes, transmission power, etc. Examples of multiple access systems include a TDMA (Time Division Multiple Access) system, a CDMA (Code Division Multiple Access) system, an FDMA (Frequency Division Multiple Access) system, an OFDMA (Orthogonal Frequency Division Multiple Access) system, an SC-FDMA (Single Carrier-Frequency Division Multiple Access) system, etc. The TDMA system uses time as radio resources, the FDMA system uses frequencies as radio resources, the CDMA system uses codes as radio resources, and the OFDMA system uses sub-carriers and time as radio resources. Furthermore, the wireless communication systems support bidirectional communication. Bidirectional communication can be performed using a TDD (Time Division Duplex) mode, an FDD (Frequency Division Duplex) mode, an H-FDD (Half-Frequency Division Duplex) mode, etc. According to the TDD mode, uplink transmission and downlink transmission are divided by time resources. According to the FDD mode, uplink transmission and downlink transmission are divided by frequency resources. According to the H-FDD mode, uplink transmission and downlink transmission are divided by combination of time resources and frequency resources.
A wireless communication system includes a base station that provides service to a cell which is a certain area. The quality of a signal transmission is affected by a variation of wireless environments in terms of characteristic of wireless transmission technology. Particularly, radio channels vary with time due to various types of scatters and movement of mobile stations. Furthermore, receive power abruptly decreases as a distance between wireless communication subjects increases, and thus the distance is restricted. Accordingly, a mobile station can communicate with a base station when the mobile station is located in the coverage of the base station, in general. As described above, a maximum transmission rate between a mobile station and a base station, throughput of a user in a cell and throughput of the entire cell decrease due to scatters, the moving speed of the mobile station, a distance between transmitting and receiving sides, etc. For example, if the mobile station is located on the boundary of the cell or an obstacle such as a building exists between the mobile station and the base station, the quality of communication between the mobile station and the base station may be poor.
To solve the aforementioned problem, various techniques for compensating for degradation of transmission signals between the base station and the mobile station are introduced to improve a maximum transmission rate and throughput and extend the coverage of the base station. One of the various techniques is introduction of a relay station in a wireless communication system. For instance, LTE (Long Term Evolution)-Advanced that is one of potent candidates of IMT (International Mobile Telecommunication)-Advanced corresponding to a next-generation mobile communication system following third-generation mobile communication systems includes a relay station. The institute of electrical and electronics engineers (IEEE) 802.16 standard which provides a technique and protocol for supporting broadband wireless access also considers the relay station. The relay station relays signals between a base station and a mobile station. The relay station can extend the coverage of the base station and improve cell throughput.
To allocate radio resources, a wireless communication system performs scheduling by considering a channel state, a buffer state, a quality of service (QoS) requirement, etc. FIG. 1 is a flow diagram showing a scheduling method for downlink data transmission between a BS and an MS. Herein, the MS may include at least one connection.
Referring to FIG. 1, the BS and the MS exchange a signal for connection configuration (step S100). A QoS requirement is included in the signal exchanged between the BS and the MS. For example, the QoS requirement may be a time delay requirement.
The MS reports a channel state of each connection to the BS (step S110). The MS may report the channel state to the BS periodically or aperiodically. Herein, the channel state may be indicated by a channel quality indicator (CQI), a signal to noise ratio (SNR), a signal to interference and noise ratio (SINR), etc.
The BS calculates a scheduling metric by considering the QoS requirement, the channel state, etc (step S120). The scheduling metric is obtained by quantifying feasibility of radio resource allocation for each MS or each connection. When the channel state between the BS and the connection is good, it implies that a large amount of data can be transmitted between the BS and the connection. Therefore, for a connection having a good channel state, the scheduling metric can be determined to be large. In addition, when the QoS requirement is high, for example, when a low time delay is required, the scheduling metric can be determined to be large.
The BS allocates a radio resource for downlink data transmission by referring to the scheduling metric (step S130). For example, the radio resource may be allocated to a connection for which the scheduling metric has a maximum value.
FIG. 2 is a flow diagram showing a scheduling method for uplink data transmission between a BS and an MS. Herein, the MS may include at least one connection.
Referring to FIG. 2, the BS and the MS exchange a signal for connection configuration (step S200). A QoS requirement may be included in the signal exchanged between the BS and the MS. The MS reports a buffer state of each connection to the BS (step S210). The BS calculates a scheduling metric by considering the QoS requirement, the buffer state, etc (step S220). For a connection for which a large amount of data is stored in a buffer, the scheduling metric can be determined to be large. The BS allocates a radio resource for uplink data transmission by referring to the scheduling metric (step S230). For example, the radio resource may be allocated to a connection for which the scheduling metric has a maximum value.
The aforementioned scheduling method is for data transmission between the BS and the MS in a situation where an RS does not exist. Therefore, a wireless communication system using the RS cannot use the scheduling method. Accordingly, there is a need for a method of effectively scheduling radio resources in the wireless communication system using the RS.